Study of percentage biased Differential relay.

Experiment No.: – 06

Objective: –

To perform the testing of the percentage biased differential relay.

Theory: –

The percentage biased differential relay has two coils, namely operating coil and
restraining coil. The operating coil is energized by the differential current (ID) whereas the
restraining coil is energized by the through (fault) current, i.e., the average current flow through the protected equipment. It is also known as bias current (IB).

Figure 1. Percentage differential relay scheme
Figure 2. Characteristic of percentage differential relay  
\[\color\red{I_D= I_1-I_2}\] \[\color\red{I_B=\dfrac{I_1+I_2}{2}}\]

where I1 and I2 are currents at two end of the protected equipment.

     The relay operates when operating coil torque is greater than restraining torque. Therefore, the current required for relay operation varies with respect to bias current. It means that the operating current is represented as some percentage of bias current, thereby it is called percentage biased differential relay. The characteristics of the percentage is represented as slope as shown in Figure 2. The slope starts from initial current setting (Is). The relay operates when the following condition is satisfied.   

\[\color\red{I_D ≥ K.I_B + I_B}\]

Circuit Diagram: –

Precaution: –

  1. Main supply is in OFF condition.
  2. Make sure that variac is in initial condition.

Procedures: –

  1. Keep the rheostat (R1 and R2) in maximum position.
  2. Turn on the switch S1.
  3. Adjust the variac to center position, thereby differential current is zero.
  4. Vary the variac linearly from center to minimum value of either side till relay operates.
  5. Note down the corresponding ammeter reading.
  6. Repeat the steps from 3 to 5 for different bias current by adjusting the rheostat R1 and R2 position equally.
  7. Repeat the steps from 3 to 6 for different slope setting.
  8. Draw the observed bias characteristic (Draw the plot for ID Vs IB).

Observation Table: –

Initial current setting (Is) =_________ ; Slope setting = ___________.

I1I2I3 (IDM)IBM= (I1+ I2)/2IDE%Error = (IDM-IDE)/IDM
Table No. 6.1

Initial current setting (Is) =_________ ; Slope setting = ___________.

I1I2I3 (IDM)IBM= (I1+ I2)/2IDE%Error = (IDM-IDE)/IDM
Table No. 6.2

Note:

IDM – Measured differential current;

IBM – Measured bias current;

IDE – Expected ID corresponding to IB; IDE to be calculated.
*Draw the plot for ID Vs IB

Calculation: –

Conclusion: –

To be written by student.

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